Understanding Unearthed: Fossil redefines knowledge about early human evolution and dispersal

A completely intact skull of an early Homo individual has been uncovered in Dmanisi, Georgia. The fossil, which is the best preserved adult early human skull to have ever been found, has completely remoulded  anthropologist’s understanding of species diversity in our early ancestors.

The archaeological site at Dmanisi, Georgia, has previously yielded early hominid skulls but never before has an entirely preserved cranium, together with its lower jaw bone, been found. The fossil is from the early Pleistocene period and inhabited the area approximately 1.8 million years ago, documenting the emergence of Homo out of Africa. Structurally, cranial features of the skull are larger and more prominent than that of previous fossils found in Dmanisi, suggesting that it belonged to a male.

The skull displays morphological features that have never before been observed together. Its small braincase coupled with comparatively large face and jaws distinguish it from all previous cranial Homo fossils that have been found. The fossil has many features that were previously used for defining different proposed Homo species. If the face and braincase of the fossil had been uncovered separately, it is likely that they would have been thought of as belonging to two separate species.

Previous to the find, the facial morphology of early Homo was best represented by two adolescent fossils with underdeveloped features, as well as one senile individual from Dmanisi whose jaws and teeth had been severely modified due to disease. This new fossil enables us to view how the face and lower jaw were positioned relative to the braincase in early Homo. Body mass and stature estimates from the fossil show the first evidence that early adult hominids had small brains but relatively large body mass and skeletal proportions, which are towards the lower limit of current human variation.

The skull, along with previous samples from Dmanisi, has established variation that indicates a possible single evolving lineage of early Homo, rather than the many different species currently proposed by many researchers. Together, the samples found at Dmanisi exhibit normal variation between sexes, and also between individuals, not dissimilar to the variation found in a modern group of humans or apes. The comparative differences in skull shape of the Dmanisi samples are similar to that found between chimpanzee individuals today. This adds to the growing evidence that variation in Homo fossils from the Pleistocene has been misinterpreted as species diversity rather than diversity within a species.

Hominid fossils from the same period, the Pleistocene, in Africa show morphological diversity often thought to represent different species. When looked at with this new evidence from Dmanisi, it appears likely that this is just variation between individuals of the same species, namely Homo erectus. The population that inhabited Dmanisi in the Early Pleistocene, around 1.8 million years ago probably originated from a single evolving lineage of Homo erectus from Africa. Subsequently, hominid specimens from Dmanisi which were previously thought to represent a different potential species of early Homo, Homo ergaster, have now been classified as a sub-species of Homo erectus.

The samples from Dmanisi reinforce the fact that early hominids migrated from Africa to Eurasia and eventually established large populations in Western Asia. However, due to the small braincase found in these samples, we know that this dispersal, likely to be the earliest of the hominids, pre-dated any significant increase in brain-size and therefore intelligence. Instead, it is likely to be features such as the use of tools that enabled increase of dietary meat, changing morphologies of the legs and feet, and increased cooperation between individuals that increased rates of mobility, reproduction and survival of Homo erectus, enabling stable populations outside of Africa to establish.

Ultimately, this newly uncovered skull has provided much needed insights into the evolutionary history, geographical dispersal and population dynamics of our early hominid ancestors. It has been a key finding in the study of human evolution and has redefined Homo erectus as the first global species of human.

Original article: D. Lordkipanidze, M. S. Ponce de Leon, A. Margvelashvili, Y. Rak, G. P. Rightmire, A. Vekua, C. P. E. Zollikofer. A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo. Science, 2013; 342 (6156): 326-330.


Gone Batty – the evolutionary origin and adaptive radiation of Chiroptera


Bats, the order Chiroptera, are among the most abundant and diverse groups of living mammals. They occupy a wide range of ecological niches in almost all environments on Earth and are highly specialised for a vast variety of different diets and habitats. There is now in excess of 1200 species of bat, and this number is growing constantly with the discovery of new species as well as the division of those already named thanks to new genetic data. Bats are the only mammals to have evolved true flight and one of only two mammalian orders that echolocation has evolved in. The evolutionary origin of bats is still somewhat unknown due to the limited fossil record of their ancestry, and their phylogeny has been, and still is, widely debated by biologists. Recent molecular studies however, have shed more light on the early radiation of Chiroptera, and this essay will discuss the current consensus of the scientific community on the matter. We will also explore the fossil record in order to ascertain the most likely direct ancestors of the order, as well as looking at the adaptive radiation and specialisation of extant groups.
Traditionally, bats have been classified into two sub-orders, these being Megachiroptera and Microchiroptera. Although it has now been recognised that these are not actually true phylogenetic groups, they still represent the two major types of bats based on morphological features alone, and so a definition of these terms is still somewhat useful in that respect. Megachiroptera, often referred to as fruit bats, are unable to echolocate (with the exception of one species which will be mentioned later in the text) and are either nectarivorous or frugivorous, as the name suggests. Microchiroptera are often called echolocating bats, as unlike fruit bats they can generate ultrasonar calls for echolocation, or insectivorous bats, although many feed on other food sources rather than insects. Bats have long been considered to be monophyletic, however Pettigrew (1986) challenged this when he proposed that Megachiroptera did not form a sister group with Microchiroptera, because of differences in their neuroanatomy. He instead advocated that the fruit bats were more closely related to primates than they were to other bats. Although published in a leading journal, this was much disputed due to the fact that if true, Pettigrew’s ‘Flying Primate Hypothesis’ would mean that flight had evolved independently in these two groups but in no other mammals. This hypothesis continued to be the subject of much debate for the rest of the 20th century, but it is now generally agreed that the evidence for bat monophyly outweighs that for Pettigrew’s hypothesis. However, molecular studies have shown that although Chiroptera form a monophyletic group, microbats are paraphyletic. Genetic evidence suggests that the superfamily Rhinolophidea (horseshoe bats and similar species), which were previously included in the Microchiroptera, are in fact polyphyletic (Teeling et al. 2002). Studies show that some rhinolophoid families in fact more closely related to the fruit bats (Pteropodidae) than they are to other bats, with which they share the ability to echolocate with (Springer et al. 2001; Teeling et al. 2005). This has led to two new groups being proposed: the Yinpterochiroptera, consisting of fruit bats and three families of echolocating bats (Rhinolophidae, Rhinopomatidae and Megadermatidae), and the Yangochiroptera, consisting of all other bat families (Teeling et al. 2002). These new groupings can be more easily visualised in Figure 1.


Figure 1. Adapted from Teeling et al. 2005.
Phylogenetic tree showing the molecular time scale of the evolutionary radiation of order Chiroptera. The x-axis shows time (m.y.a.). Circles indicate the age of the oldest specimen in the fossil record that represents that particular lineage. ‘K-T boundary’ refers to the approximate time of the Cretaceous-Paleogene, associated with the K-T mass extinction event.

The generally most accepted time scale for the evolution of bat families was proposed by Teeling et al.  2005, as shown in Figure 1. The study used both molecular and morphological data, and showed the four major insectivorous bat lineages (Rhinolophidea, Emballonuroidea, Noctilionoidea and Vespertilionoidea) appearing between 50 and 52 million years ago. This diversification coincided with a global rise in temperature and an increase in insect diversity, which may have resulted in the diversification of these lineages due to the new food sources available to them (Teeling et al. 2005).

It is likely that the direct ancestors of bats were small arboreal insectivores, nocturnal and lacking the ability of flight (Gunnell & Simmons 2005). Molecular phylogenies support the placement of bats within Laurasiatheria (Pumo et al. 1998), the Northern superorder of mammals, suggesting that they aren’t as closely related to primates and treeshrews as morphological evidence once led us to believe. However, genetic evidence has not yet been able to determine the true sister group of bats, with Eulipotyphlans (shrews, moles and hedgehogs) or Cetferungulates (cetaceans, ungulates and carnivora) being the two major propositions for this (Van Den Bussche & Hoofer 2004; Nishihara et al. 2006). The fossil record of Chiroptera is limited compared to other mammalian orders as bat skeletons are fragile and do not preserve well. In addition to this, most specimens that we do have are not old enough to tell us anything about the origins of the order, and therefore the early evolution of bats is not yet well understood. The oldest bat fossils that have been found date back to the early Eocene, and by the mid-Eocene bats we know that bats had already diversified greatly, with many fossil specimens representing modern bat families (Gunnell & Simmons 2005). The two oldest fossil bats, Onychonycteris and Icaronycteris, coexisted approximately 50 million years ago, however Onychonycteris was by far the most primitive of the two. Figure 2 shows both of these fossils alongside a skeleton of the extant insectivorous bat, Myotis lucifugus for comparison.


Figure 2. A comparison of skeletons (in dorsal view) of the two earliest known bats with an extant insectivorous species.

a) Onychonycteris finneyi. Source: Simmons et al. 2008.
b) Icaronycterus index.  Source: Simmons & Geisler 1998.
c) Myotis lucifugus (Little brown bat). A common extant insectivorous bat of North America. Photograph by M. A. Wilson (College of Wooster, OH, U.S.A), used with permission.

Whereas Icaronycterus and all other later fossil bats closely resemble modern insectivorous bats, Onychonycteris exhibits more primal features. Onychonycteris has limb proportions that are intermediate between those of extant bats and those of all other, non-flying, mammals, suggesting it was more agile and not capable of as powerful flight as its modern counterparts (Simmons et al. 2008). Onychonycteris also possessed claws on all five of its forelimb digits, rather than the maximum of two claws we see in all other extinct and modern bats, so it was likely an intermediary stage between these bats and their terrestrial predecessors (Simmons 2008). Debate is still rife as to whether flight or echolocation evolved first in bats, and Onychonycteris seemed to provide the answer to this when first found, as its ear morphology including its relatively small cochlea provided evidence for the assumption that it could not echolocate (Simmons et al. 2008; Teeling 2009). However, Veselka et al. 2010 disputed this, suggesting a way that Oynchonycteris could have echolocated and also noting that the specimen in question had been flattened during fossilisation, meaning that the exact structure of the skull could not be determined for definite.

Although it is not known whether flight or echolocation evolved first, or even if the evolution of both features coincided, biologists are fairly unanimous when it comes to considering the reasons as to why bats evolved them. Ancestors of bats are thought to have had membranes between their limbs, such as the ones in modern flying squirrels, which enabled them to glide before they developed fully functioning wings.  As these early mammals were arboreal, gliding would have been advantageous because it would have conserved some of their energy when moving between trees, as well as enabling them to avoid terrestrial predators.  The evolution of the bat wing seems to have happened very rapidly, and it has been associated with a single protein’s (Bmp2) expression (Sears et al. 2006). The increased agility (as seen in Onychonycteris) and the new gliding abilities of these early nocturnal ancestors of bats would have meant that their orientation skills needed to be improved in order for them to successfully hunt in the night sky. It is highly probable that echolocation in bats became increasingly more sophisticated as they became better at flying (Altringham 2011). As previously mentioned, it is now widely accepted that bats are indeed a monophyletic group, but this leads us to two different hypotheses about the evolution of echolocation within this group. Either echolocation evolved in both the Yangochiroptera and the Rhinolophidae, or it had one origin in bats and was then lost in the Pteropodidae (see Figure 1). Regardless of which is true, echolocation then evolved in the Egyptian fruit bat (Rousettus aegyptiacus), however as a less sophisticated process using tongue-clicks to produce ultrasound calls, rather than the larynx as all other echolocating bats do (Jones & Teeling 2006).

Extant bat species have diversified greatly, and whereas the single family of fruit bats, Pteropodidae, are restricted to tropical Asia and Africa, echolocating bats occur in all habitats except for the polar regions. Evolving the ability to echolocate and therefore detect and track prey in complete darkness has provided bats with the opportunity to fill the largely unoccupied niche of the night sky. Pteropodidae are exclusively herbivorous, but echolocating bats are far more diverse in their feeding habits, deriving their nourishment from many different sources such as nectar, pollen, fruit, small vertebrates, and even blood, as well as the obvious insects. It is likely that these variations on food sources originated from early bats displaying different foraging strategies, just as their modern relatives do now. As well as catching prey in the air, they probably also gleaned insects from surfaces such as plants and fruit. Exploitation of the resources near to these surfaces (e.g. nectar, pollen, and fruit) may have eventually led to specialisation and the switching of food sources in some species (Altringham 2011). Many echolocating bats are highly specialised for a particular food source, however an example of a bat species with a particularly derived feature is Desmodus rotundus, the common vampire bat. Vampire bats feed on mammalian blood, and this species has been found to have heat receptors in its nose which help it locate the areas of skin where blood flows closest too, enabling more efficient feeding (Schafer et al. 1988).

The most diverse family within the Chiroptera are the leaf nosed bats (Phyllostomidae) This is likely to be at least partly due to the fact that Phyllostomids evolved in the neotropics, which are by far the largest forests in the world, meaning that more space was available for many more species to be supported within. Both the neotropics and the Old World tropics are located close to the equator, and this may have been a contributing factor of the radiation of bat species which originated here, as species diversity is known to increase with distance to the equator (Willis & Whittaker 2002).

As we have seen in this essay, bats form the most diverse, specialised, and arguably interesting mammalian order extant today. The vast range of ecological niches they occupy no doubt contribute to their abundance all over the world and their success that has spanned at least the last 50 million years. The early evolution of Chiroptera is still poorly understood due to the limited fossil record; however, the increasing ability for molecular techniques to be employed in order to study genetic data as well as morphological features, is greatly enhancing our understanding of the adaptive radiation of bats.


Continue reading “Gone Batty – the evolutionary origin and adaptive radiation of Chiroptera”

Does sex make sense?

Sex is risky. In the short term, it even seems detrimental to survival. So then why do most organisms reproduce sexually rather than asexually?

Was it the lack of sex that led to Paranthropus robustus‘ demise?

Reproduction – the process of producing offspring from one or more “parent” organisms – is essential for all life on Earth and without it no species would be able to survive over time. The majority of organisms reproduce sexually, meaning that they produce gametes – cells that contain half of an individual’s genetic information – which need to be fertilised by another gamete in order to grow into a new individual. Sexual reproduction most often involves two individuals of different sexes, each contributing one gamete. However, some organisms such as Angiosperms reproduce by self-fertilisation, which is the fusion of two gametes which have been produced by the same individual. Asexual reproduction is the process whereby all prokaryotes, as well as protists and some other eukaryotes, produce offspring by creating genetically identical copies of themselves. There are various different ways of reproducing asexually, the most common being fission, which is when a cell replicates its DNA and then divides so that two or more daughter cells are formed from the original cell. Budding, another form of asexual reproduction, is carried out by organisms including the yeast fungi and the animal genus Hydra. Budding involves a small growth developing on a parent cell. The parent cell’s nucleus containing its genetic information then divides, and one section migrates into the newly formed daughter cell. The daughter cell continues to grow while attached to the parent cell until it has matured, when it then detaches from the parent cell and can reproduce by budding itself.

For most multicellular species, sexual reproduction has one main advantage over asexual reproduction, and this is the fact that it creates variation in the population. This is explained by Mendel’s fundamental laws of heredity; both chromosomes during meiosis and gametes during fertilisation are independently assorted, meaning that when cells reproduce sexually, any combination of chromosomes from the two parents could be inherited. When gametes are produced, each one will receive either the paternal or maternal copy of each chromosome. The chromosomes that are passed on to each gamete are randomly selected, so every chromosome in a gamete is of equal likelihood to have been inherited from either parent. This ensures that meiosis produces two daughter cells that are genetically different to each other, and their parents.

Meiosis also involves recombination, which is another contributing factor to increasing variation. It involves two homologous chromosomes crossing over and exchanging genetic information. Recombination ensures that the chromosomes a daughter cell inherits will not usually be an exact copy of one of the parent’s chromosomes, as the alleles of some genes linked to each chromosome will have been exchanged on to its homologue. This means that it is highly unlikely that siblings will inherit exactly the same genetic information; although their genotype can be anywhere from 0% to 100% similar, these extremes are very rare and siblings most often share approximately 50% of their genetic information.

The creation of variation is highly important as it is essential to the survival of most species and also drives evolution. Variation ensures that individuals of a species are all different and this makes the species much more likely to establish and survive over time. If there is a widespread disease affecting a population with variation in their genotypes, it is likely that some individuals will carry an allele that makes them resistant to the disease. For example, when the bubonic plague spread in the 14th century, the majority of survivors in Europe had a mutation in an allele that gave them resistance to the pandemic. As a result, most of the current population of Northern Europe now carry this mutation in their genomes. Conversely, if individuals of a species all shared the same genotype, a disadvantageous mutation such as one that impacted functionality or survival would spread through the population extremely quickly and soon affect the majority of individuals of that species.

Sexual reproduction does have its disadvantages for organisms, and these are mainly linked to the increased chance of contracting a disease through sexual rather than asexual reproduction. Sexually transmitted diseases are widespread throughout the animal kingdom, and range from bacterial infections similar to those found in humans, to parasites such as Coccipolipus hippodamia, small mites that live on their ladybird hosts and whose larvae infect the host’s mate during intercourse.

Chance of disease in many animals is also increased due to sexual reproduction because of their sex determination systems – in most mammals and some insects this is the XY chromosome system, and in birds and various other animals it is the ZW chromosome system. In the XY sex determination system, a zygote will either inherit an X chromosome from their mother and a Y chromosome from their father meaning it will develop into a male (XY), or it will inherit an X chromosome from each parent and develop into a female (XX). The ZW chromosome system works in much the same way, except the female is the heterogametic sex. A small part of each of the X and Y chromosomes, called the pseudoautosomal region, are homologous, which allows the chromosomes to pair and segregate properly in meiosis in males. However, the rest and vast majority of these chromosomes cannot pair and therefore cannot recombine to exchange genetic information. This increases the likelihood of X-linked disorders becoming more abundant in the population. In humans, recessive X-linked disorders are the most common; if a woman carrying the allele for the disorder becomes pregnant her sons will have a 50% chance of inheriting the gene and therefore being affected by the disorder, and her daughters will have a 50% chance of inheriting the gene and being carriers for the disorder. Y-linked disorders are extremely rare in humans, but they theoretically have the ability to spread very rapidly through a population, as although no females would be affected, a male with a Y-linked disorder would pass the deleterious allele on to all of his sons. The Y chromosome has evolved to counteract this by eroding almost all of the genes that were once linked to it, except those with a function that is specifically related to the male sex. As a result, the Y chromosome is very small compared to other chromosomes, so there is less chance of a deleterious mutation to occur on it. This also means that if a mutation does occur, it is likely to affect a gene that has an essential function in the male reproductive system. So any individual with a Y-linked disorder will in all probability be infertile and therefore unable to pass the deleterious mutation on to any offspring. Some insects, such as crickets and grasshoppers have an XO sex determination system, whereby offspring that inherit two X chromosomes will become female and those which inherit just one will develop into males. These invertebrates share a common ancestor which used the XY system, so it is thought that the Y chromosome has degraded so much over time it has been lost. It is possible that this is due to selection pressures from Y-linked diseases in these organisms, and perhaps this is what is slowly happening with the erosion of the Y chromosome in other organisms.

Asexual reproduction has many advantages and one of these is that offspring can be produced very quickly, without the need for individuals to invest time and energy into finding a mate, copulating, or rearing their young. When conditions are favourable, some strains of bacteria can reproduce as quickly as every 20 minutes, meaning a single individual could produce a population of millions in less than 24 hours. Large colonies of bacteria can be produced in a very short period of time, and the larger the colony, the more chance it has of outcompeting others and ultimately surviving. The fact that asexual reproduction produces clones of the original organism also has its advantages. As offspring will be a genetically identical copy of their parent organism, they will share the same fitness and will be able to reproduce just as the original organism did. There is no risk of fitness decreasing, unlike there is in sexual reproduction due to half the offspring’s genetic information being provided by another individual.

Organisms that reproduce asexually tend to be smaller and less complex than those that undergo sexual reproduction to produce offspring. This is largely due to the fact that there is less scope for natural selection to act upon organisms that reproduce asexually; the genetic information of a population will be exactly the same, so the only cause of variation in an individual will be through mutations in its DNA. Because of the lack of recombination in the genetic material, although an advantageous allele would spread through the population quickly, there is less chance of one individual being significantly fitter than the rest. If a deleterious mutation occurred, this would also spread quickly through generations and the whole population would be affected by it. Similarly, if a disease affected one individual in a population, it would affect them all as the lack of variation in the genotype means that none of the organisms in the population would be resistant. This could quickly wipe out an entire population.

For the majority of higher organisms, sexual reproduction seems to be the best strategy as it creates variation in the population and makes the species as a whole more likely to survive. However, this seems paradoxical as in evolutionary terms, asexual reproduction is more advantageous – an individual’s fitness increases with their reproductive success, which will be normally be higher for asexual organisms as their offspring are identical genetic copies of themselves. Organisms that reproduce asexually will produce offspring who too will be able to reproduce as they share the same fitness. Although sexual reproduction is more advantageous for the long term survival of the species, evolution is only driven by enhancing a single organism’s fitness in the present. The question to why a large portion of organisms have evolved to reproduce sexually when evolution should select for increased fitness in the present time and not look ahead to the survival of a whole species remains unanswered. In conclusion, it seems that asexual reproduction is more suited to smaller organisms who need to be able to produce offspring quickly and efficiently or perhaps cannot move around enough to find a mate, whereas sexual reproduction is highly advantageous for higher organisms whose offspring need a significant amount more time to grow and develop due to their size and complexity.